Numerous variable speed A.C. motor systems have been proposed heretofore, and some of these systems are in use, but the use of the better systems is restricted because they are quite complex and extremely expensive. Less expensive A.C. motors are in use for machine tools and for moderate horsepower applications, wherein variable speed is obtained by commutating wound rotors with brush contacts feeding A.C. potential from a variable resistance connected to an A.C. power supply. This last type of motor is quite inefficient, the losses being greatest at the lower speeds, and the response is somewhat erratic, and other problems thereof, such as high brush-commutator maintenance, are well known to those in the art.
There is a wide need for a relatively simple, reliable, inexpensive, efficient and readily controllable variable speed A.C. motor having no commutator. Such a motor would enable more economical and efficient operation of many devices and apparatus, as well as securing more desirable modes and better functioning of such devices and apparatus for their respective fields of service. The term "variable speed" is employed herein with reference to the normal or full running speed of the motor.
As an illustration of such need, most fans, blowers, water and fluid pumps, ventilating and air-conditioning systems employ an A.C. motor operating at one set speed which is pre-selected to produce a certain maximum flow of a fluid--air, water or gas--needed for a designed output demand on that system. If the demand on the system falls below this designed demand level, then the usual mode thereof is to run the motor intermittently by starting and stopping it in response to a thermostat or other control. Such intermittent operation is not only inefficient and wasteful of energy, but is unsatisfactory because, in the case of an air conditioner, it delivers a blast of extra cold air into a room for a brief period and then stops so that no air circulates and the room occupant soon begins to feel warm and may become uncomfortable before the unit restarts the motor and another blast of cold air is delivered. Such intermittent operation with noisy and quiet periods is not only annoying but may be aggravating.
An air-conditioner, or other apparatus, would operate far more efficiently if it ran continuously at a moderately quiet speed to circulate at all times a gentle flow of cool air to maintain a room at a desired comfortable temperature, using a motor that would vary its speed to accomodate need for either higher amounts of cooling or less cooling. A controllably variable speed motor has another great advantage in that it can operate for periods of time at speeds above a normal maximum speed to meet unusual demands, as for instance when an extremely hot spell occurs or a large group meets in an air-conditioned room. A conventional one-speed motor cannot exceed its normal maximum speed and consequently has no capacity for operation above its rated speed.
A variable speed motor operable over a wide range of speeds would also give greatly improved efficiency in the operation of air blowers in aircraft, which at present usually employ one-speed motors. Because air pressure varies so greatly between sea level and flying altitudes, blower output and efficiency is poor at one time or another. A variable speed motor will operate the blower efficiently at slow speeds, at sea level, and will speed up to deliver a desired volume of air at high flying altitudes, also with good efficiency of both motor and blower.